Introduction to array and multidimensional array usage in Python

Add A.append (' a ') on it. As long as the order is added, there is no problem.
When used, the subscript can be used completely:

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A[0] A[1]

An exception is thrown if the result references a subscript that does not exist. At this point, you need to add elements first, then the reference will be no problem. If you want to reserve space in advance, you can use loops to give the list, a default value for each element, and then reference will not be a problem.
Such as:

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A=[] For I in range (100): A.append ([]) For j in Range (100): A[i].append (0)

This generates an array with a 100*100 default value of 0.

#创建一个宽度为3, array with a height of 4
#[[0,0,0],
# [0,0,0],
# [0,0,0],
# [0,0,0]]
myList = [[0] * 3] * 4 But when the operation mylist[0][1] = 1 o'clock, the whole second column is found to be assigned, and becomes

[[0,1,0],

[0,1,0],

[0,1,0],

[0,1,0]]

Why... I don't understand, back to the Python Standard Library to find the answer

List * N->n shallow copies of list concatenated, the connection of a shallow copy of n list

Cases:

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>>> lists = [[]] * 3 >>> lists [[], [], []] >>> Lists[0].append (3) >>> lists [[3], [3], [3]] [[]]

is a list containing an empty list element, so [[]]*3 represents 3 references to this empty list element, modifying any

An element changes the entire list:

So you need to create multidimensional arrays in a different way to avoid shallow copies:

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>>> lists = [[] for I in range (3)] >>> Lists[0].append (3) >>> Lists[1].append (5) >>> Lists[2].append (7) >>> lists

[[3], [5], [7]] The two-dimensional array was created in the following way:

MyList = [([0] * 3) for I in range (4)]

For example, to dynamically create a two-dimensional array:

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int** Ptwodimarr = new INT[10][20]; Ptwodimarr[0][0] = 5;

This seems intuitive, but the compilation can be an error:

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Cannot convert from ' int (*) [~] ' to ' int * * *

If so:

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int ptwodimarr[10][20]; Ptwodimarr[0][0] = 5;

is OK, but the memory of this array is allocated on the stack, so

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void Main () { int ptwodimarr[1000][1000]; }

No problem compiling, running will die

It's okay to be a global variable, but it's best to stop it.

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int ptwodimarr[1000][1000]; void Main () { }

Such

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int (*ptwodimarr) [1000] = new int[1000][1000]; Ptwodimarr[0][0] = 5;

Dynamic allocation is possible, one can remember to release memory, second dimension or fixed (first dimension variable), is not a bit uncomfortable

Because C + + is a "static" language, in the compile period to determine everything, so in the definition of multidimensional arrays in addition to the first dimension variable, the other several dimensions are fixed to constant, because the array in memory is linear storage.